Power MOSFETs (metal-oxide-semiconductor field-effect transistors) comprise one of the most useful field effect transistors implemented in both analog and digital circuit applications as energy saving switches.
In general, a trench-based power MOSFET is built using a vertical structure as opposed to a planar structure. The vertical structure enables the transistor to sustain both high blocking voltage and high current.
The conventional trench MOS transistors have realized much higher cell density than the planar MOS transistors. But the denser pitches and the trench structures have increased gate-drain overlap capacitances and gate-drain charges. At high density the resistance of these structures is mainly limited by the epitaxial resistance for a given breakdown voltage. The so called split gate structure was proposed to overcome several drawbacks of the conventional trench structure performance. In this structure a shielded poly, which is connected to the source, is placed under the gate poly inside the trench.
Split gate structures have been known to have better switching, breakdown voltage, and lower on-resistance characteristics. But due to its complexity the split gate structure is more difficult to manufacture. Also at high density it is necessary to bury the split gate structure under a top isolation oxide so that a space saving self-aligned contact technique can be utilized. Under these conditions difficulties in forming a isolation oxide, gate poly, inter poly oxide and shield poly inside a trench are very challenging.